Lung injury risk assessment during blast exposure

Blast pulmonary trauma are common consequences of modern war and terrorism action. To better protect soldiers from that threat, the injury risk level when protected and unprotected must be assessed. Knowing from the literature that a possible amplification of the blast threat would be provided by some thoracic protective systems, the objective is to propose an original approach to correlate a measurable parameter on a manikin with a pulmonary risk level. Using a manikin whose response is correlated with the proposed tolerance limits should help in the evaluation of thoracic protective system regarding injury outcomes.A database including lung injury data from large mammals have been created, allowing the definition of iso-impulse tolerance limits from no lung injury to severe ones (∼60% of ecchymosis). As the use of this metric is not sufficient to evaluate the performance of protective systems on a manikin, the iso-impulse tolerance limits were associated with the thoracic response of post-mortem swine under blast loading. It was found that the lung injury threshold in terms of incident impulse is 58.3 kPa·ms, corresponding to a chest wall peak of acceleration/velocity/displacement of 7350 m/s², 3.7 m/s and 6.4 mm respectively. Lung injuries are considered as severe (30–60% of ecchymosis) when the incident impulse exceed 232.8 kPa·ms, leading to a chest wall peak of acceleration/velocity/displacement of 79.7 km/s², 14.7 m/s and 30.1 mm respectively.The defined lung tolerance limits are valid for a 50 kg swine (unprotected) exposed side-on to the blast threat and against a wall.     

Fringe mode transmittance laser Doppler microscope anemometer: its adaptation for measurement in the microcirculation

Blood flow analysis in the microcirculation requires accurate measurement of velocity, volume flow and shear-rate versus shear-stress relationships. The resolution of most anemometers is too limited to obtain useful measurements, especially near the blood vessel wall and at branches and bifurcations. To make such measurements possible with a noninvasive, high resolution, accurate technique, we have developed a fringe mode, transmittance laser Doppler microscope anemometer (LDMA). This system has an intrinsically high spatial resolution (10 × 12 μm), and does not require a high concentration (10⁶/cm³) of scatterers or red blood cells (RBC) as in our application. Preliminary measurements of water flow in a rectangular channel were conducted to ascertain the reliability and accuracy of velocity measurements using the LDMA. Velocity profiles were then measured by the LDMA system in arterioles 38–135 μm in diameter, in the transparent, everted cheek pouch of the anaesthelized hamster. The extremely high resolution of the optical system, and the ultra-fine traversing mechanism of the microscope slage, made velocity readings larger than 0.02 mm/s with accuracy and reproducibility better than 1%, possible near the wall to within 7–10 μm.     

Chest response assessment of post-mortem swine under blast loadings

To better protect soldiers from blast threat, that principally affect air-filled organs such a lung, it is necessary to develop an adapted injury criterion and, prior to this, to evaluate the response of a biological model against that threat. The objective of this study is to provide some robust data to quantify the chest response of post-mortem swine under blast loadings.7 post-mortem swine (54.5 ± 2.6 kg), placed side-on to the threat and against the ground, were exposed to 5 shock-waves of increasing intensities. Their thorax were instrumented with a piezo-resistive pressure sensor, an accelerometer directly exposed to the shock-wave and a target was mounted on the latter in order to track the chest wall displacement.For incident impulses ranging from 47 kPa ms ± 2% to 173 kPa ms ± 6%, the measured maximum of linear chest wall acceleration (Γmax) goes from 5800 m/s² ± 16% to 41,000 m/s² ± 8%, with a duration of 0.8 ms. Chest wall displacements ranging from 5 mm ± 20% to 20 mm ± 15%, with a duration of 9 ms, are reached. These reproducible data were used to find simple relations (linear, 2nd and 3rd order polynomials) between the kinematic parameters (plus the viscous criterion) and the incident and reflected impulses.Correlating the new reproducible data with the prediction from the Bowen curves showed a lung injury threshold in terms of Γmax similar to that of Cooper (10,000 m/s²). However, the limits defined for the viscous criterion in the automobile field and for non-lethal weapons seems not adapted for the blast threat.     

Laser assessment of leaflet closing motion in prosthetic heart valves

The dynamics of leaflet motion in heart valve prostheses (HVP), and in particular the closing velocity, is believed to be related to the valve sound and possibly to the phenomenon of valve cavitation. This paper describes a non-intrusive laser sweeping technique enabling the study of leaflet motion. The principle of measurement and the equipment involved are presented, together with the results of two commerially available, 29 mm bileaflet mitral valves, a St. Jude Medical, and an Edwards Duromedic valve. Experiments were carried out in a pulsatile mock flow testing loop designed to mimic physiological pressure waveforms and ventricular contraction. Measurements of heart rate were made in the range 70–120 beats min⁻¹, with a ventricular pressure slope range of 1800–5600 mm Hgs⁻¹ and a cardiac output range of 5.0–7.5 litres min⁻¹. Motion analysis of the measured data focuses on the velocity of the leaflet immediately before closure.     

Plasma chemistry of the sealed-off slab CO laser active medium pumped by radio-frequency discharge with liquid-nitrogen-cooled electrodes

The long-term time behavior of the output power of a sealed-off cryogenic slab CO laser pumped by a repetitively pulsed RF discharge and operating on the overtone (λ = 2.6–3.5 μm) vibrational?rotational transitions of the CO molecule was studied experimentally. It is shown that adding of an anomalously large amount of oxygen (up to 50% with respect to the CO concentration) to the initial gas mixture CO : He = 1 : 10 leads to a manyfold (by several tens of times) increase in the duration of the laser operating cycle (until lasing failure due to the degradation of the active medium). In this case, the laser life-time without replacement of the active medium reaches 10⁵–10⁶ pulses. Using various diagnostics (including luminescence spectroscopy and IR and UV absorption spectroscopy), regularities in the time-behavior of the concentrations of the main component of the active medium (CO molecules) and the products of plasmachemical reactions (O₃, CO₂) generated in the discharge gap during the laser operating cycle are revealed. Time correlation between the characteristics of the active medium and the laser output power are analyzed. A phenomenological approach to describing the entirety of plasmachemical, purely chemical, gas-dynamic, and diffusion processes determining the behavior of the laser output characteristics throughout the laser operating cycle is offered.     

Instrumentation for measurement of dental plaque thickness in situ

A commercial digital micrometer has been modified mechanically and electronically to allow the measurement, in situ, of the thickness of dental plaque. The device detects initial contact between a moving probe and the plaque, and measures subsequent probe displacement through to the tooth surface. Instrument accuracy is ± 5 μm over a displacement range of 0–5 mm, with 1 μm resolution. In practice, after a short ‘learning’ phase, reliable clinical results can be obtained.     

Study of Electric Explosion of Flat Micron-Thick Foils at Current Densities of (5−50)×10<Superscript>8</Superscript> A/cm<Superscript>2</Superscript>

Electric explosions of flat Al, Тi, Ni, Cu, and Та foils with thicknesses of 1?16 μm, widths of 1?8 mm, and lengths of 5?11 mm were studied experimentally on the BIN, XP, and COBRA high-current generators at currents of 40?1000 kA and current densities of (5–50) × 10⁸ A/cm². The images of the exploded foils were taken at different angles to the foil surface by using point projection radiography with an X-pinch hot spot as the radiation source, the spatial resolution and exposure time being 3 μm and 50 ps, respectively, as well by the laser probing method with a spatial resolution of 20 μm and an exposure time of 180 ps. In the course of foil explosion, rapidly expanding objects resembling the core and corona of an exploded wire were observed. It is shown that the core of the exploded foil has a complicated time-varying structure.     

FIBROBLAST CELL KINETICS IN THE PERIODONTAL LIGAMENT OF THE MOUSE

Twelve male mice were injected intraperitoneally with tritiated thymidine. Six were sacrificed after 1 hr and six after 7 days. The right manibular incisors were dissected, cut sagittally and dipped in liquid emulsion. In the exposed and stained slides, observation was restricted to the lingual side of the periodontal ligament. Cells were evaluated sagittally from the basal tooth end up to the distance of 5 mm and up to the depth of 100 μm in the direction of socket wall. Cell and grain count was evaluated separately in 100 × 10 μm rectangles, creating a two-dimensional array onto which the periodontal ligament was mapped. The progenitor compartment extends up to the distance of 2400 μm from origin. Fibroblasts leaving this compartment migrate at different velocities, creating a velocity profile across the ligament. Adjacent to the socket wall cell movement is sluggish, whereas the fastest cell movement is exhibited by cells located 20–30 μm from the tooth. The existence of such a profile indicates a continuous renewal of intercellular bonds, consistent with a process of actively pulling the incisor from its socket by the migrating fibrocytes.     

Confocal measurement of the three-dimensional size and shape of plant parenchyma cells in a developing fruit tissue

Parenchyma cells from the inner mesocarp of a grape berry (Vitis vinifera L. cv. Chardonnay) were visualised in three-dimensions within a whole mount of cleared, stained tissue using confocal laser scanning microscopy and digital image reconstruction. The whole berry was fixed, bisected longitudinally, cleared in methyl salicylate, stained with safranin O and mounted in methyl salicylate. Optical slices were collected at 1.0 μm intervals to a depth of 150 μm. Neighbouring z-series were joined post-collection to double the field-of-view. Attenuation at depth of the fluorescent signal from cell walls was quantified and corrected. Axial distortion due to refractive index mismatch between the immersion and mounting media was calibrated using yellow-green fluorescent microspheres and corrected. Transmission electron microscopy was used to correct fluorescent measurements of cell wall thickness. Digital image reconstructions of wall-enclosed spaces enabled cells to be rendered as geometric solids of measurable surface area and volume. Cell volumes within the inner mesocarp tissue of a single grape berry exhibited a 14-fold range, with polysigmoidal distribution and groupings around specific size classes. Cell shape was irregular and the planes of contact were rarely flat or simple. Variability in cell shape was indicated by the range in surface area to volume ratios, from 0.080 to 0.198 μm^–1. Structural detail at the internal surface of the cell wall was apparent. The technique is applicable to a wide range of morphometric analyses in plant cell biology, particularly developmental studies, and reveals details of cell size and shape that were previously unattainable.     

The video technique developed for measuring epicardial strains on guinea pig's left ventricle

Single-plain video used for measurements of epicardial strains is a technique that yields minor interference with the studied mechanical properties of the ventricle. Due to its low temporal resolution, the existing technique is, however, not appropriate for small animals. We questioned whether the technique could be improved enough to cope with higher heart rates and miniaturization necessary for experiments on rats, mice and guinea pigs. Therefore, we developed a high-speed video system and used it for measuring epicardial strains in guinea pig hearts in situ with the open chest. The improvement was achieved in video hardware (camera: Dalsa D6-0256; framegraber: EPIX PIXCI D32) and software, the markers (glowing acrylate crystals; diameter approximately 0.15 mm) and illumination (UVA light, OSRAM L). Three markers were attached onto the epicardium in the equatorial region of the left ventricular free wall, 1.5 mm apart, with fibrin glue. From their coordinates, we calculated two-dimensional finite strains with end diastole as the reference point. The accuracy of the displacement measurement of the technique and the error introduced by approximate-visual estimation of the left ventricle coordinate system were evaluated. The accuracy of the displacement measurement was +/-1.6 microm and the temporal resolution was 2 ms. Error due to approximate coordinate system orientation was +/-3% of the strain amplitude. The typical amplitude of strains was -0.06, -0.11 and 0.04 in circumferential, axial direction and in-plane shear, respectively. The improvements enable us to perform physiologically relevant measurements of epicardial deformations on guinea pig heart.     

Heart position variability during voluntary moderate deep inspiration breath-hold radiotherapy for breast cancer determined by repeat CBCT scans

Voluntary moderate deep inspiration breath hold (vmDIBH) in left-sided breast cancer radiotherapy reduces cardiac dose. The aim of this study was to investigate heart position variability in vmDIBH using CBCT and to compare this variability with differences in heart position between vmDIBH and free breathing (FB).For 50 patients initial heart position with respect to the field edge (HP-FE) was measured on a vmDIBH planning CT scan. Breath-hold was monitored using an in-house developed vertical plastic stick. On pre-treatment CBCT scans, heart position variability with respect to the field edge (Δ_HP-FE) was measured, reflecting heart position variability when using an offline correction protocol. After registering the CBCT scan to the planning CT, heart position variability with respect to the chest wall (Δ_HP-CW) was measured, reflecting heart position variability when using an online correction protocol. As a control group, vmDIBH and FB computed tomography (CT) scans were acquired for 30 patients and registering both scans on the chest wall.For 34 out of 50 patients, the average HP-FE and HP-CW increased over the treatment course in comparison to the planning CT. Averaged over all patients and all treatment fractions, the Δ_HP-FE and the Δ_HP-CW was 0.8 ± 4.2 mm (range −9.4–+10.6 mm) and 1.0 ± 4.4 mm (range −8.3–+10.4 mm) respectively. The average gain in heart to chest wall distance was 11.8 ± 4.6 mm when using vmDIBH instead of FB. In conclusion, substantial variability in heart position using vmDIBH was observed during the treatment course.     

Ultrastructure of a Coccidium (Apicomplexa: Sporozoea: Coccidia) in Priapulus caudatus (Priapulida)1

Stages in the life cycle of a coccidium are described from the intestine of Priapulus caudatus Lamarck, 1816. Meronts, merozoites, microgamonts, microgametes, and walled and unwalled macrogametes were seen in intestinal cells. Meronts were about 8 μm long and 3–7 μm wide and produced up to seven merozoites. Free merozoites were about 9 μm long and 4 μm wide and contained about 43 subpellicular microtubules that terminated in the outer polar ring. Microgamonts were up to 23 μm long and 7 μm wide and usually were delimited by a single membrane. Microgametes were about 5 μm long, exclusive of the two flagella, about 2 μm wide, and contained a nucleus that was not uniformly dense. Macrogametes, about 6 μm in diameter, had a nucleus largely without dense chromatin. The oocyst wall formed around intracellular macrogametes to a thickness of 0.2–0.5 μm as thin, osmiophilic elements that became arranged in reticular and tubular layers. Wall-forming bodies were not seen, but fine filaments may participate in wall formation, as these were found between the outer membrane of the pellicle and the nearest wall elements. Microgametes and walled macrogametes were delivered to the lumen of the host intestine during apocrine secretion or excretion by the intestinal cells. Fertilization may occur in the intestinal lumen. Unsporulated ovoid oocysts, 18–27 μm long and 10–14 μm wide, with a 3 μm micropyle and a wall 0.6–0.7 μm thick, were passed from the host.     

Précautions,pièges et artéfacts en TEP/TDM du thorax

PET/CT with ¹⁸F-FDG is a test widely used in the evaluation of chest diseases as diverse as oncology indications and in particular the evaluation of pulmonary nodules, mediastinal disorders of the chest wall, breast lesions or even heart diseases including those of infectious valves. Despite the many precautions taken to the successful completion of the examination, a caution of the interpretation of the exam is required, given the pitfalls and artifacts inherent to the technique, respiratory and cardiac movements, the misrepresentation lesions (false positives of false negatives) or altered by the treatment. Their knowledge can minimize their impact on the interpretations and avoid the use of supplements investigations, sometimes invasive, or worse, administering a noxious or inappropriate treatment. The purpose of this paper is to describe the pitfalls and artifacts of PET/CT with ¹⁸F-FDG commonly found in the thoracic region but also to expose the various precautions that can help the conduct of the exam, the interpretation and even the optimal patient management.     

Lanthanide binding to cardiac and skeletal muscle microsomes. Effects of adenosine triphosphate, cations, and ionophores.

Lanthanide (gadolinium, Gd) binding to cardiac and skeletal muscle microsomes was studied, and high- and low-affinity sites were identified. The high-affinity constant was 10⁶ M^?1, and there were 131 and 107 nmol/mg bound to this site in dog heart and rabbit skeletal muscle, respectively. Zn²⁺, Cd²⁺, Al³⁺, and Ca²⁺ (5 mm) inhibited binding, especially of the high-affinity site. Ionophores X537A (10 μm) and A23187 (1–2 μm) increased lanthanide binding and did not cause release. Addition of ATP in low concentration (20–50 μm) increased the binding of Gd without hydrolysis of the ATP. The extra binding induced by ATP was blocked by heating the microsomes and was reversed by [ethylenebis(oxyethylenenitrilo)]tetraacetic acid. High concentrations (10^?4–10^?3, m) of ATP blocked extra Gd binding by competitive chelation. The Ca²⁺-activated ATPase was inhibited by Gd and stimulated by X537A. The Gd did not block the ionophore-stimulated increase in Ca²⁺-ATPase activity. It is postulated that lanthanides bind predominantly to the ionophoric component of the Ca-transport site rather than the hydrolytic site and that ATP may facilitate such binding without being split.     

Development of Ox-Coyote Cycle of Sarcocystis cruzi1

The ox-coyote cycle of Sarcocystis cruzi was studied by killing 38 calves between 4 and 153 days postinoculation (DPI) with 55 × 10³-5 × 10⁸ sporocysts from the intestines of coyotes. At 4 DPI, a zoite was found within the lumen of a mesenteric lymph node artery. At 7 DPI, zoites were found in mononuclear cells and in endothelial cells in mesenteric arteries. First generation meronts (41.0 × 17.5 μm in diameter) occurred 7–26 DPI in mesenteric lymph nodes. At 19–46 DPI, second generation meronts occurred in kidneys, muscles, and other tissues: renal meronts were 19.6 × 11.0 μm, and intramuscular meronts were 25.0 × 11.1 μm. Merozoites were found in the peripheral blood 17 DPI and later at 24–46 DPI. They divided by endodyogeny in mononuclear cells. Sarcocysts were seen first in the heart at 45 DPI and contained one or two metrocytes. At 55 DPI, sarcocysts containing only metrocytes were found in striated muscles, heart, and in smooth muscles of the urinary bladder, rumen, omasum, abomasum, and small intestine. At 67, 87, 112, and 153 DPI, sarcocysts were found only in striated muscles and in the heart. At 67 DPI, sarcocysts were up to 360 μm long. They contained only metrocytes and were not infective to the dog. At 86 DPI, sarcocysts contained mostly bradyzoites, a few metrocytes, and were infective to a coyote. The thin-walled sarcocysts grew to a maximum length of 800 μm and contained bradyzoites that were 10.9 × 3.0 μm. At 90 DPI, two mature sarcocysts were found in 2 of 73 sections of brain and spinal cord; hundreds of sarcocysts were present in sections of tongue and heart of this calf. Gametogony occurred in the small intestine of the coyote. Macro-and microgamonts were found in goblet cells of the small intestines of coyotes 6 h after the ingestion of infected meat. Microgamonts were few and contained 3–11 slender gametes. Oocysts were seen at 12 h and sporulation was completed 9 DPI. The prepatent period in the coyote was 8 days. The ox-coyote cycle is compared with ox-dog cycle.     

Comparative morphology and cytology of the eye,with particular reference to the retina,in lizardfishes (Synodontidae,Teleostei)

Fishelson, L., Delarea, Y. and Goren, M. 2012. Comparative morphology and cytology of the eye, with particular reference to the retina, in lizardfishes (Synodontidae, Teleostei). —Acta Zoologica (Stockholm) 93 : 68–79. The retinas of nine species of lizardfishes (Synodontidae) are composed of double cones, single cones, and rods. The cones are 16–28 μm long, and their number in the fundus of adult Synodus variegatus reaches ca. 32,900 mm² (varying from ca. 300,000 to ca. 390,000 in a 10 mm² of the retina), while in Saurida spp., they number ca. 12,000–14,000/mm². The cone ellipsoids are with up to 600 mitochondria, 0.5–1.6 μm in diameter. The rods are 30–50 μm long; their outer segments 0.6–1.2 μm thick and 15–18 μm long; their inner segments elongated. Their number varies from 15 to 128 million/retina. In fish of similar dimensions but of different species, the number of visual cells in the retina differs. In all species, the eyes increase from 2.0 mm in diameter in the smallest fish studied to 12 mm in the largest one. With eye growth, the retina in the various species increases from ca. 3.8 mm² in the smallest fish to ca.160.0 mm² in the large Saurida macrolepis. The possible ecological aspects of the observed phenomena are discussed.     

Sizes of bacterial cells in soils determined by cascade filtration technique

This paper studies the number of bacteria in typical chernozem and mountain-meadow soil by the traditional method and the cascade filtration technique. The total number of bacteria in these soils, which was obtained in filters of different diameters during filtering the suspension of a certain amount, is 1.5–5 times higher than that obtained by the traditional method. In the structure of the bacterial biomass in both soils, the biomass of bacterial cells with a diameter of 0.38–0.43 μm was dominating by 8–90%. In the typical chernozem, the biomass of cells with a diameter of 0.17 μm was slightly more than 1%; in the mountain-meadow soil, the percentage of the biomass of cells with a diameter of 0.17 μm increased by 5%. The average volume and diameter of the bacteria in the studied soils were calculated. In typical chernozem, the average volume of bacterial cells was equal to 0.0046 μm³ and the diameter was 0.206 μm. In the mountain-meadow soils, these values were slightly lower, 0.0038 μm³ and 0.194 μm, respectively. The biomass of the bacterial cells, which is usually calculated based on the cell volume of 0.1 μm³, is overestimated by about five times when counting the number on the filters. The percentage of the real biomass of soil bacteria is traditionally much lower than that estimated.     

Dispersal of Septoria nodorum Pycnidiospores by Simulated Raindrops in Still Air

Simulated raindrops, diameter c. 3 or 4 mm, fell 13 m down a raintower onto suspensions of Septoria nodorum pycnidiospores, depth 0.5 mm, or infected straw pieces. Splash droplets were collected on pieces of fixed photographic film. It was estimated that one drop generated c. 300 spore carrying splash droplets, containing c. 6000 spores, from a concentrated spore suspension (6.5 × 10⁵ spores/ml) and c. 25 spore-carrying droplets, containing c. 30 spores, from infected straw pieces (11 × 10⁶ spores/g dry wt). When the target was a spore suspension in water without surfactant, most spore-carrying droplets were in the 200—400 μm size category and most spores were carried in droplets with diameter >1000 μm. When surfactant was added to spore suspensions, most spore-carrying droplets were in the 0–200 μm category and most spores were carried in droplets with diameter 200–400 μm and none in droplets >1000 μm. Regression analyses showed a significant (p < 0.001) relationship between square root (number of spores per droplet) and droplet diameter; the slope of the regression line was greatest when surfactant was added to the spore suspensions. The distribution of splash droplets with distance travelled from the target was better fitted by an exponential model than by power law or Gaussian models. The distributions of spore-carrying droplets and spores with distance were fitted better by an exponential model than by a power law model. Thus regressions of log, (number collected) against distance were all significant (p < 0.01); the slopes of the regression lines were steepest when surfactant was added to the spore suspension. At a distance of 10 cm from target spore suspensions most splash droplets and spore-carrying droplets were collected at height 10–20 cm, with none above 40 cm; at a distance of 20 cm there were most at heights 0–10 cm and 40–50 cm.     

Glucose-6-phosphate dehydrogenase from Leuconostoc mesenteroides. Interaction of the enzyme with coenzymes and coenzyme analogs.

Glucose-6-phosphate dehydrogenase from Leuconostoc mesenteroides utilizes either NAD⁺ or NADP⁺ as coenzyme. Kinetic studies showed that NAD⁺ and NADP⁺ interact with different enzyme forms (Olive, C., Geroch, M. E., and Levy, H. R. (1971) J. Biol. Chem.246, 2047–2057). In the present study the techniques of fluorescence quenching and fluorescence enhancement were used to investigate the interaction between Leuconostoc mesenteroides glucose-6-phosphate dehydrogenase and coenzymes. In addition, kinetic studies were performed to examine interaction between the enzyme and various coenzyme analogs. The maximum quenching of protein fluorescence is 5% for NADP⁺ and 50% for NAD⁺. The dissociation constant for NADP⁺, determined from fluorescence quenching measurements, is 3 μm, which is similar to the previously determined K_m of 5.7 μm and K_i of 5 μm. The dissociation constant for NAD⁺ is 2.5 mm, which is 24 times larger than the previously determined K_m of 0.106 mm. Glucose 1-phosphate, a substrate-competitive inhibitor, lowers the dissociation constant and maximum fluorescence quenching for NAD⁺ but not for NADP⁺. This suggests that glucose 6-phosphate may act similarly and thus play a role in enabling the enzyme to utilize NAD⁺ under physiological conditions. When NADPH binds to the enzyme its fluorescence is enhanced 2.3-fold. The enzyme was titrated with NADPH in the absence and presence of NAD⁺; binding of these two coenzymes is competitive. The dissociation constant for NADPH from these measurements is 24 μm; the previously determined K_i is 37.6 μm. The dissociation constant for NAD′ is 2.8 mm, in satisfactory agreement with the value obtained from protein fluorescence quenching measurements. Various compounds which resemble either the adenosine or the nicotinamide portion of the coenzyme structure are coenzyme-competitive inhibitors; 2′,5′-ADP, the most inhibitory analog tested, gives NADP⁺-competitive and NAD⁺-noncompetitive inhibition, consistent with the kinetic mechanism previously proposed. By using pairs of coenzyme-competitive inhibitors it was shown in kinetic studies that the two portions of the NAD⁺ structure cannot be accommodated on the enzyme simultaneously unies they are covalently linked. Fluorescence studies showed that there are both “buried” and “exposed” tryptophan residues in the enzyme structure.